With the advancement in microprocessor technology and better understanding of pharmacodynamics and pharmacokinetics of anaesthetic agents, computer facilitated closed loop control of anaesthesia using propofol has been shown to be accurate with better performance than manual control. Literature on computer controlled administration of inhalational anaesthetics is few, as it requires the computer to control the dial setting on the vapouriser. The investigators intend to compare the computer controlled closed loop administration of isoflurane by infusing it into the anaesthetic circuit with conventional vaporiser control in elective open heart surgery.

40 patients (ASA (American Society of Anesthesiology) class II-IV; 18- 65 years) undergoing elective cardiac surgery requiring cardiopulmonary bypass (CPB) will be randomly divided into manual or closed loop groups. Propofol will be used for induction of anaesthesia in both groups followed by isoflurane for maintenance. In the manual group, isoflurane will be administered through the Tech 7 vapouriser during pre and post CPB periods to target bispectral index (BIS) of 50. In closed loop group, isoflurane will be administered using infusion of liquid isoflurane into expiratory limb of the closed circuit. This rate of infusion though a conventional syringe pump will be controlled by algorithm termed 'Improvised Anaesthetic Agent Delivery System' (IAADS) to maintain BIS of 50. Patients in both groups will receive 500ml of 100 % oxygen as fresh gas flow. The % of time bispectral index (BIS) is within the 10 of set target BIS of 50 will be the primary outcome measure. The secondary outcome measures will be median performance error (MDPE)(2), median absolute performance error (MDAPE)(2), wobble(2), divergence(2), amount of isoflurane used and hemodynamic parameters will be secondary outcome measures.

The duration of time depth of anesthesia was maintained in the recommended range (as measured by BIS) during the period isoflurane (general anesthetic) was administered to the study population. This value expressed as percentage is the primary outcome. BIS is an objective measure of depth of anesthesia derived from statistical(bispectral) analysis of electroencephalographic waves. BIS ranges from 0 to 100. It decreases monotonically from 100 in the awake state to lower values with sedation and anesthesia.

The difference between the observed and target of measure of depth of anesthesia (BIS) expressed as percentage of target BIS is calculated as performance error every 30 seconds. This value may be either '+' or '_' indicating whether the observed measure is above the target (overshoot-+) or below the target (undershoot-_). The median value of all performance errors during isoflurane anesthesia is median performance error and is a measure of bias of the system. This outcome is expressed as the mean of Median Performance Errors per participant.

The median of the absolute values of performance errors (without considering the direction of error) is median absolute performance error. This outcome measures the magnitude of error or inaccuracy of the system studied. A lower value indicates a more precise system.This outcome is expressed as the mean of Median Absolute Performance Errors per participant.

Wobble measures the intra-individual variability in performance error.The median of the difference between individual performance errors throughout anesthesia and the median performance error for each participant is the wobble of that participant. The mean value per participant is indicated in the outcome measure.

The duration of time heart rate remained within 25% of the pre-operative baseline value during the period isoflurane (general anesthetic) was administered to the study population. This value is expressed as a percentage. This outcome is expressed as the mean of percentage of time per participant.

The duration of time mean arterial pressure remained within 25% of the pre-operative baseline value during the period isoflurane (general anesthetic) was administered to the study population. This value is expressed as percentage. This outcome is expressed as the mean of percentage of time per participant

In the manual group isoflurane was administered using Tech 7 vapouriser. The dial setting was controlled by the anesthesiologist to achieve and maintain a BIS of 50 during anesthesia.

Other: Conventional control

The isoflurane administration to deliver anesthesia will be done conventionally through a Tec 7 vaporiser to maintain the bispectral index at 50.

Other Name: MANUAL CONTROL

Experimental: IAADS group

In this group, liquid isoflurane was injected into the circuit using a syringe pump controlled by the IAADS system.The IAADS system has the algorithm to regulate the rate of infusion of isoflurane such that BIS is achieved and maintained at 50 during anesthesia.

Other: Closed loop control

The isoflurane administration to maintain anesthesia will be by infusion to the anesthesia circuit using syringe pump (injection technique). The rate of delivery of isoflurane will be regulated by computer that uses a control algorithm.

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Please refer to this study by its ClinicalTrials.gov identifier: NCT01069562